Method for fabricating silicon oxynitride

ABSTRACT

A method for making silicon oxynitride comprising providing a vaporous gas stream of a compound selected from the group consisting of silazanes and siloxazanes. An enclosed, heated reaction site is also provided. The vaporous gas stream is delivered to the enclosed, heated reaction site in which the levels of oxygen are strictly controlled to promote the formation of silicon oxynitride particles.

This application is a 371 of PCT/US98/16358 filed Aug. 5, 1998 nowWO99/11513.

FIELD OF THE INVENTION

The present invention relates to nitrogen doped silica, which may alsobe called silicon oxynitride or SiO_(x)N_(y). More particularly, thepresent invention relates to nitrogen doped silica formed by usingsilazane or siloxazane starting materials.

BACKGROUND OF THE INVENTION

Silicon oxynitride is used in a variety of applications. The ability tovary the refractive index of silicon oxynitride over a wide range makesit an attractive material for optical applications. The refractive indexof pure SiO₂ is 1.46, and the refractive index of Si₃N₄ is 2.1.Therefore, the refractive index of silica doped with nitrogen can bevaried between 1.46 and 2.1. In addition. doping silica opticalwaveguides with nitrogen helps to prevent UV radiation damage to thewaveguide which causes undesirable losses.

In optical waveguide applications, silicon oxynitride has been producedby plasma and nonplasma CVD processes, using silane and/or ammoniagases. For optical applications, however, use of ammonia is undesirablebecause ammonia contains hydrogen, and the resulting synthesized siliconoxynitride may contain a substantial proportion of hydrogen whichsignificantly contributes to losses in the waveguide.

In addition, silane raw materials must be handled very carefully due tothe violent reaction caused when air is introduced into a closedcontainer of silane. Silane is typically used in producing thin films onsemiconductor substrates, which requires the deposition of a film havinggood characteristics for semiconductor applications. In the manufactureof semiconductor thin films the properties of the film are moreimportant than deposition rate. In the production of optical devices,however, large quantities of material must be produced quickly, and thedeposition rates for producing optical devices such as opticalwaveguides are much faster than deposition rates for semiconductor thinfilms.

Silicon oxynitride may also be produced by the pyrolysis or hydrolysisof organometallic halides such as silicon tetrachloride. However, use ofhalides is not favored because the pyrolysis and hydrolysis of thesematerials produces chlorine or a very strong acid by-product,hydrochloric acid (HCl). Hydrochloric acid is detrimental not only tomany deposition substrates and to reaction equipment but also is harmfulto the environment.

Additionally, it is difficult to produce bulk silicon oxynitride andwaveguide preforms using conventional outside vapor deposition (OVD)processes, which expose the deposited material to air. One difficultyencountered in forming silicon oxynitride using conventional OVDprocesses is that when processing occurs in a system open to air, oxygenatoms preferentially bond to silicon atoms over nitrogen atoms, formingsilica instead of silicon oxynitride.

In a typical OVD process, a carrier gas is bubbled through a liquidorganic silicon containing compound. The resulting vaporous compound istransported to a burner via a carrier gas, wherein the vaporous gasstreams are combusted in a burner flame fueled with natural gas andoxygen. The presence of oxygen in conventional OVD processes convertsthe vaporous reactants to their respective oxides, exiting the burnerorifice to form a stream of volatile gases and finely-divided sphericalparticles of soot that may be deposited onto a substrate forming aporous blank or preform of soot, for example, silica soot.

U.S. Pat. No. 5,152,819 to Blackwell et al., the disclosure of which isincorporated by reference, describes the use halide-free siliconcontaining compounds including octamethylcylotetrasilazane in an OVDprocess to produce high purity fused silica glass.Octamethylcyclotetrasilazane, [(CH3)2SiNH]4, hereinafter referred to asOMCTSZ, is a white solid at room temperature and has a boiling point of225° C. An OVD process described in U.S. Pat. No. 5,152,819, which usedOMCTSZ as a feedstock for the process produced a pure silica soot withless than 0.01% nitrogen contained in the soot.

In view of the difficulties encountered in manufacturing siliconoxynitride, there is an explicit need for a method for manufacturingsilicon oxynitride which avoids the aforementioned problems.Specifically, it would be desirable to provide a method formanufacturing silicon oxynitride which does not contain a substantialproportion of hydrogen. In addition, it would be desirable to provide aprocess which avoids the preferential bonding of oxygen atoms to siliconatoms, which results in the formation of pure silica.

SUMMARY OF INVENTION

Applicants have discovered a method for manufacturing silicon oxynitridecomprising the steps of providing a vaporous gas stream of a compoundselected from the group consisting of siloxazanes and silazanes. As oneexample of processing a compound in accordance with the method of thepresent invention, solid octamethylcyclotetrasilazane (OMCTSZ) isheated, preferably to a temperature of about 130° C. to about 225° C.,to provide OMCTSZ liquid, and a vaporous gas stream may be provided bybubbling an inert carrier gas through the OMCTSZ liquid to create avaporous OMCTSZ gas stream. The vaporous silazane gas stream isdelivered to an enclosed reaction site which is heated to a temperatureof at least about 500° C., preferably between 700° C. and about 900° C.,where the gas stream is converted into particles of silicon oxynitridecontaining greater than 0.1% nitrogen by weight.

In an important aspect of the invention, the amount of oxygen present atthe reaction site is strictly limited to prevent formation of puresilica at the reaction site and to promote the formation of siliconoxynitride. Preferably the level of oxygen at the reaction site islimited to very low levels by controlling the partial pressure of oxygenin the enclosed reaction site. The amount of oxygen present at thereaction site will depend on the desired composition of the siliconoxynitride end product produced by the method of the present invention.The stream of vaporous silazane forms silicon oxynitride at the heatedreaction site. In an alternative embodiment, the stream of vaporoussilazane gas can be combined with a vaporous gas stream of a siliconcontaining compound such as octamethylcyclotetrasiloxane.

Thus, the present invention provides a method for manufacturing siliconoxynitride which does not contain a substantial proportion of hydrogenand provides a method which avoids the preferential bonding of oxygenatoms to silicon atoms encountered in OVD processes. Additional featuresand advantages of the invention will be set forth in the descriptionwhich follows, and in part will be apparent from the description. or maybe learned by practice of the invention. It is to be understood thatboth the foregoing general description and the following detaileddescription are exemplary and explanatory and are intended to providefurther explanation of the invention as claimed.

DETAILED DESCRIPTION

Reference will now be made in detail to a preferred embodiment of theinvention. The present invention provides a method of manufacturingsilicon oxynitride using silazane or siloxazane starting materials.

The present invention provides a method of manufacturing siliconoxynitride comprising the steps of providing a vaporous gas stream of asiloxazane or a silazane and delivering the vaporous gas stream to anenclosed reaction site, which is heated to a temperature of at leastabout 500° C. As used in this application, the term “silazane” means anorganosilicon nitrogen compound containing one or more silicon-nitrogenbonds, including amino silazanes, linear silazanes, and cyclosiloxanes,wherein a nitrogen atom and a single element or group of elements arebonded to the silicon atom. The term “siloxazane” as used in thisapplication are compounds containing the unit [O—Si—N], including linearand cyclic siloxanes. A variety of silazanes and siloxazane may be usedin the method of the present invention, including polysilazanes andpolysiloxazanes.

Delivery of the vaporous silazane or siloxazane gas stream to thereaction site may be accomplished by using an inert carrier gas such asnitrogen, argon, or helium. Advantageously, the amount of oxygen presentat the reaction site is strictly limited to prevent the formation ofsilica. The amount of oxygen at the reaction site is limited bycontrolling the partial pressure of the oxygen in the enclosed reactionsite.

In an exemplary embodiment, a vaporous silazane gas stream is providedby heating solid octamethylcyclotetrasilazane (OMCTSZ) to provide OMCTSZliquid. The solid OMCTSZ should be heated to at least about 120° C.,preferably to about 140° C., to melt the OMCTSZ to its liquid state. Thesolid OMCTSZ may be contained in a vessel and heated with any suitableheat source such as a hot plate, an oil bath, or heat tape. The methodmay further comprise bubbling an inert carrier gas through the OMCTSZliquid to create a vaporous OMCTSZ gas stream. As used in thisspecification “inert gas” means a nonreactive gas, such as argon,nitrogen. or helium. The vaporous OMCTSZ is then delivered to anenclosed reaction site heated to about 700° C. to about 800° C., wherethe amount of oxygen is strictly controlled to promote the formation ofSiO_(x)N_(y) particles containing greater than 0.1% nitrogen by weight.

The enclosed reaction site may be, for example, a fused silica tube. Thetube may be placed in a furnace to heat the reaction site or the tubemay be surrounded by a heating element or a flame. By sealing the tube.the amount of oxygen inside the tube may be controlled. The vaporousOMCTSZ gas may be delivered by into the tube by a mass flow controller.

The oxygen present at the reaction site may be controlled several ways.For example, delivering oxygen to the reaction site via a mass flowcontroller enables control of the amount of oxygen in the composition ofthe final SiON product. Limiting the amount of oxygen present at thereaction site promotes the formation silicon oxynitride and prevents theformation of pure silica. The composition of the silicon oxynitrideproduced by the process of the present invention can be varied accordingto the desired end use of the material. The material may, for example,be used for optical waveguide applications. and the amount of nitrogenin the silicon oxynitride composition would depend on the opticalproperties of the waveguide such as the desired refractive index profileof the waveguide. For any desired composition, the optimum flow rate ofthe OMCTSZ gas stream and oxygen gas can be determined byexperimentation.

The amount of oxygen present at the reaction site may also be limited bysimply enclosing the reaction site for example in a sealed tube, andallowing the reaction to occur with the oxygen present in the ambientair inside the tube. Thus, to form SiON, it may not be necessary tosupply any oxygen to the reactor tube. For example, in one experimentalrun, solid OMCTSZ was heated to 133° C. to form OMCTSZ liquid, and 200standard cubic centimeters per minute of nitrogen was bubbled throughthe liquid to form a vaporous OMCTSZ gas stream. The OMCTSZ gas streamwas delivered to a reaction site, which was a silica tube heated to 750°C. No oxygen was added to the reaction site. and the final SiON materialproduced contained 25.84% oxygen, as determined by electron spectroscopyfor chemical analysis (ESCA).

As mentioned above, the silicon oxynitride made by the method of thepresent invention may be used for optical waveguides. As previouslydiscussed, the refractive index of Si₃N₄ is higher than the refractiveindex of SiO₂, By doping a silica waveguide with nitrogen to formSiO_(x)N_(y), a waveguide core may be formed, over which a silicacladding may be added to form an optical waveguide. The amount ofnitrogen contained within the core material will depend on the desiredrefractive index profile of the waveguide.

If desired, the reaction stream of vaporous silazane or siloxazane canbe combined with the with reaction stream of another silicon containingorganic material such as octamethylcyclotetrasiloxane, which isdelivered to the reaction site to provide an additional silica sourcematerial. An optical waveguide preform may be fabricated by using themethod of the present invention wherein silicon oxynitride is depositedinside a fused silica tube. Thus, the silicon oxynitride depositedmaterial forms a core region having a higher index of refraction thanthe cladding region which may comprise the wall of the silica tube.

If the material formed according to the method of the present inventionis to be used as a waveguide, the processes following the formation ofthe waveguide blank would follow those practiced in industry. Inconventional practice, optical waveguide fabrication is a three-stepprocess. Most of the processes currently used for the manufacture ofoptical waveguides involve a laydown process wherein a blank ismanufactured by a CVD process such as OVD, MCVD, AVD, or PCVD. Thesecond stage of an optical fiber manufacturing process typicallyinvolves heat treating the blank in a helium/chlorine atmosphere to fullconsolidation. The third stage the blank is drawn into a waveguide suchas a waveguide fiber.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the of the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method of manufacturing silicon oxynitridecomprising the steps of: providing a vaporous gas stream of a compoundselected from the group comprising silazanes and siloxazanes; providingan enclosed reaction site heated to a temperature of at least about 500°C.; limiting the amount of oxygen present at the reaction site; anddelivering the vaporous gas stream to the reaction site without ammoniato form silicon oxynitride containing greater than 0.1% nitrogen byweight.
 2. The method of claim 1 wherein the silazane is polysilazane.3. The method of claim 2 wherein the polysilazane is acyclopolysilazane.
 4. The method of claim 3 wherein thecyclopolysilazane is octamethylcyclotetrasilazane.
 5. The method ofclaim 4 wherein the step of providing a vaporous gas stream comprisesthe steps of: heating solid octamethylcyclotetrasilazane to provideoctamethylcyclotetrasilazane liquid; and bubbling an inert carrier gasthrough the octamethylcyclotetrasilazane liquid to create a vaporousoctamethylcyclotetrasilazane gas stream.
 6. The method of claim 5wherein the carrier gas is selected from the group consisting ofnitrogen, argon and helium.
 7. The method of claim 6 wherein thereaction site is heated to a temperature of about 700° C. to about 800°C.
 8. The method of claim 7 wherein the solidoctamethylcyclotetrasilazane is heated to a temperature of at leastabout 130° C. to about 225° C.
 9. The method of claim 8 wherein theenclosed reaction site is a fused silica tube.
 10. The method of claim 9further comprising the step of combining theoctamethylcyclotetrasilazane gas stream with a vaporous gas stream of asilicon containing compound.
 11. The method of claim 1 wherein the stepof providing a vaporous gas stream comprises providing the compound in aliquid state and bubbling an inert carrier gas through the compound toform the vaporous gas stream.
 12. The method of claim 11 wherein thestep of providing a vaporous gas stream comprises providing the compoundin a solid state and heating the compound into a liquid state.
 13. Themethod of claim 11 wherein the inert carrier gas is selected from thegroup consisting of nitrogen, argon and helium.
 14. The method of claim1 wherein the compound is below its boiling point before delivery to thereaction site.
 15. A method of manufacturing silicon oxynitridecomprising the steps of: providing a vaporousoctamethylcyclotetrasilazane gas stream, including heating solidoctamethylcyclotetrasilazane into liquid octamethylcyclotetrasilazaneand bubbling an inert carrier gas through theoctamethylcyclotetrasilazane liquid; providing an enclosed reaction siteheated to a temperature of at least about 500° C.; limiting the amountof oxygen present at the reaction site; and delivering the vaporousoctamethylcyclotetrasilazane gas stream to the reaction site to formsilicon oxynitride containing greater than 0.1% nitrogen by weight. 16.The method of claim 15 wherein the carrier gas is selected from thegroup consisting of nitrogen, argon, and helium.
 17. The method of claim16 wherein the reaction site is heated to a temperature of about 700° C.to about 800° C.
 18. The method of claim 17 wherein the solidoctamethylcyclotetrasilazane is heated to a temperature of at leastabout 130° C. to about 225° C.
 19. The method of claim 18 wherein theenclosed reaction site is a fused silica tube.
 20. The method of claim19 further comprising the step of combining theoctamethylcyclotetrasilazane gas stream with a vaporous gas stream of asilicon containing compound.
 21. A method of manufacturing siliconoxynitride comprising the steps of: providing a vaporous gas stream of acompound selected from the group comprising silazanes and siloxazanes;providing an enclosed reaction site heated to a temperature of at leastabout 500° C.; limiting the amount of oxygen present at the reactionsite; and delivering the vaporous gas stream to the reaction site toform silicon oxynitride containing greater than 0.1% nitrogen by weight,wherein no ammonia is present at the reaction site.
 22. A method ofmanufacturing silicon oxynitride comprising the steps of: providing anoctamethylcyclotetrasilazane gas stream; providing an enclosed reactionsite heated to a temperature of at least about 500° C.; limiting theamount of oxygen present at the reaction site; and delivering theoctamethylcyclotetrasilazane gas stream to the reaction site to formsilicon oxynitride containing greater than 0.1% nitrogen by weight.